Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, under certain operating conditions, such as high engine speeds, increased EGR may flow to the compressor, thereby increasing the compressor speed and temperature. Degradation to the compressor may occur if a gas temperature of gases entering the compressor and/or the speed of the compressor increases above threshold levels. The inventors herein have recognized that an EGR valve disposed in the EGR passage may be closed to reduce EGR flow to the compressor, thereby decreasing a temperature of exhaust gas flowing through the compressor and a speed of the compressor. However, the inventors have also recognized that closing the EGR valve may trap hot residual gases within the cylinders and/or first exhaust manifold and may also reduce blowthrough.
In one example, the issues described above may be addressed by a method comprising: in response to an engine operating condition, decreasing gas flow from a first exhaust manifold to an intake passage, upstream of a compressor, a first set of exhaust valves exclusively coupled to the first exhaust manifold; and in response to the decreasing gas flow, increasing gas flow from the first exhaust manifold to an exhaust passage coupled to a second exhaust manifold coupled to a second set of exhaust valves. As one example, the engine operating condition may include an inlet temperature of the compressor being lower than a first threshold temperature, an outlet temperature of the compressor being above a second threshold temperature, a speed of the compressor being above a threshold compressor speed, an engine speed being over a threshold engine speed, and/or an engine load being over a threshold engine load. In this way, the engine operating condition may be a condition at which over-speed, over-temperature, and/or condensation at the compressor may occur. Thus, by decreasing the gas flow from the first exhaust manifold to the intake passage, upstream of the compressor, in response to the engine operating condition, compressor degradation may be reduced. Further, by increasing gas flow from the first exhaust manifold to the exhaust passage in response to the decreasing gas flow to the intake passage, exhaust gases and blowthrough air expelled from the first set of exhaust valves may be directed to the exhaust passage, thereby reducing pressures in the first exhaust manifold and trapping of residual gases within the cylinders.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.